When zoologists decide upon the classification of an animal, it is often easy for the nonscientist to understand their rationale. After all, is there any question that butterflies and moths are closely related? And isn't it a given that dogs and cats have enough obvious similarities to justify their taxonomic relationship? But a tiny snail and a carnivorous octopus? A giant squid and a sea scallop? A delicate oyster and a troublesome zebra mussel? What possible thread could connect these six creatures  and more than 65,000 other species  so strongly that scientists assigned them to the same phylum, the one whose members are called mollusks?

Characteristics and Classification

The common bond among all these species is the presence of a soft body. (The phylum name Mollusca is, in fact, derived from the Latin mollis, meaning "soft.") In many (but not all) mollusks, this soft body is protected by a shell secreted by the body covering known as the mantle. The shell itself is made largely of calcium carbonate.

Another common feature of many mollusks is the presence of a foot, an unusual structure that takes different forms in different species. In clams, for example, the foot is a muscular extension that the creature uses to plow its way through mud and sand. In snails, it is flat and used for creeping. In squids and octopuses, the foot is divided into "arms," which the animals use to seize prey.

The phylum Mollusca is divided into six distinct classes: Cephalopoda, which includes squids, cuttlefish, octopuses, and the chambered nautilus; Bivalvia, which includes oysters, clams, scallops, mussels, and teredos; Gastropoda, which includes snails, slugs, limpets, abalones, and conches; Scaphopoda, the tooth shells; Polyplacophora, chitons, the most primitive mollusk class; and Monoplacophora, represented by the living Neopilina and numerous extinct forms of the phylum.

Squids, Octopuses, and Their Relatives

The cephalopods (which means "head-feet" in Greek) include such striking creatures as squids and octopuses. These animals are so called because the foot, which is separated into a number of "arms," encircles the head.

Unlike most other mollusks, living cephalopods generally do not develop shells. Instead, the mantle forms the outer part of the body. In some species, there is an inner skeleton. Numerous extinct forms of cephalopods resembled the living pearly nautilus in that they formed a coiled, chambered shell.

All cephalopods dwell in the sea. They have arms, often called tentacles, that are equipped with suckers or hooks or both. Almost all cephalopods secrete an inklike fluid, which is stored in a special sac. When a cephalopod wishes to escape a pursuer, it squirts out the ink, making the water turbid and thus confusing the foe. Most cephalopods are capable of chameleon-like color changes as well. On their skin, specialized cells called chromatophores ("color-bearers") contain different pigments. When these cells expand or contract, the color of the skin changes rapidly.

Squids

The champion swimmer of the cephalopods is the squid, a streamlined, spindle-shaped creature sometimes called the "arrow squid" for its ability to dart through the water. The squid's foot is divided into 10 arms. Two of the arms are longer than the rest; these longer appendages bear suckers and are used to seize and hold prey. The eyes have no lids, but otherwise look startlingly human.

The squid draws water through a central body cavity  the mantle cavity  and forces it out through a flexible tube, the siphon, when the mantle is contracted. The siphon is located just behind the arms. The jet of water that spurts through the animal propels it swiftly backward. Ink is also discharged through this siphon.

The fins  two flaplike extensions of the mantle  function chiefly for steering and to propel the squid slowly forward or backward.

One of the most familiar species is the North American common squid, Loligo pealei, found primarily in the Mediterranean, in East Asian waters, and along the east coast of North America. Some fishermen use the creature as bait. In many parts of the world, this squid serves as human food.

Another species, the flying squid (Ommastrephes bartrami), has been compared to the flying fish. It often shoots out of the water, particularly when the weather is rough, and has been known to land on the decks of ships.

One of the most formidable species is the giant squid, Architeuthis princeps. Until recently, the giant squid was considered the largest of all invertebrates (animals without backbones), with a total length  including its body and arms  of 50 feet (15 meters) or more. But in 2003, scientists recovered an intact specimen of Mesonychoteuthis hamiltoni, dubbed "colossal squid," which exceeds by more than 10 percent the size of the largest known adult A. princeps. The rare M. hamiltoni, an aggressive predator equipped with two razor-sharp beaks and swiveling talonlike barbs at the ends of its tentacles, lives far beneath the surface, and is rarely encountered by seafarers.

Octopuses

Few dwellers of the deep stir the human imagination the way the octopus (genus Octopus) does. Many tales have been told of these creatures attacking hapless waders or divers. Such tales are grossly exaggerated  and probably not true. Certainly, a large octopus  with its eight long, powerful arms, its two large staring eyes, and a vicious-looking beak  would be a rather unpleasant creature to encounter underwater. But however threatening an octopus may appear, there is scant evidence to suggest that even the largest species bother  let alone attack  humans.

The foot of the octopus is divided into eight arms, the feature that gives rise to its name, which means "eight feet" in Greek. The animal has a parrotlike beak with which it rends its prey. Octopuses range from 2 inches to 30 feet (5 centimeters to 9 meters) in arm-and-body length. The larger species, sometimes called "devilfish," may attain a weight of 77 pounds (35 kilograms). The octopus can crawl along the sea bottom on its arms. Sometimes the creature swims about by sucking water into the body and then squirting it out.

Most octopuses are shy and retiring, passing their days hidden in crevices. At nightfall, they set out in search of prey. Stealthily, an octopus creeps up on some unsuspecting fish or crab. Once the powerful arms entwine the victim, there is no escape. The beaklike jaws quickly end the captive's struggle, and the octopus feeds upon the prey. With the approach of dawn, the animal retreats to its lair. The octopus is itself the prey of eels, whales, and sharks.

Humans dine on octopus in coastal areas of Europe, North America, in various parts of the Far East, and on the islands of the South Pacific.

Cuttlefishes and Nautiluses

A close relative of the squid is the common sepia, or cuttlefish, Sepia officinalis. This small creature, ranging from 6 to 10 inches (15 to 25 centimeters) in length, secretes a calcareous inner shell known as cuttlebone. Pieces of this substance are often placed in birdcages for the pet bird to peck at  and thereby derive its required allotment of certain minerals. Another cuttlefish product is sepia, a pigment prepared from the deep brown fluid (ink) that the creature ejects as a defense mechanism.

The pearly, or chambered, nautilus (genus Nautilus), found in the South Pacific and the Indian Ocean, is a member of an ancient group of cephalopods. Its shell is spirally coiled and divided into compartments, each one a chamber in which the nautilus once lived at some stage of its growth; the animal resides in the outermost chamber. About 90 tentacles are set around the mouth. Although the tentacles lack suckers, they can nevertheless cling tenaciously to solid objects. The head can be withdrawn into the shell. A hood at the back of the head partly closes the opening.

The female of the paper nautilus, or argonaut, Argonauta argo (a form closely related to the octopus), secretes a spirally coiled and symmetrical white shell each year before mating. This delicate shell serves as an egg case; the argonaut can drop it at will. The female may reach a length of 8 inches (20 centimeters); the male is much smaller  only about 1 inch (2.5 centimeters) long  and never secretes a shell.

Clams and Other Bivalves

Clams, oysters, mussels, and teredos belong to the class Bivalvia, or Lamellibranchia. They are called bivalves because their shells are divided into two parts, or valves. The inner surface of the shell is coated with a substance called nacre, or mother-of-pearl. This fine-grained layer may be white or may be as multihued as a rainbow.

The two valves are joined by one or two muscles strong enough to hold the shell tightly closed; it is these muscles that are cut when a mussel or clam is opened. Some bivalves, such as clams, have a well-developed foot, which the animal extends beyond the shell to move from place to place. As adults, true oysters cannot move about; instead, they remain firmly attached to solid objects on the bottom of the sea. Bivalves lack a specially differentiated head.

Some bivalves have two tubes, or siphons, through which water is drawn in and forced out. The incoming water contains tiny organisms that serve as food: protozoans, eggs, larvae, the spores of algae, and minute plants called diatoms. Food is taken into the digestive canal by way of a mouth opening. Oxygen enters the blood through the two gills. Wastes are eliminated with the outgoing water.

Oysters

The true, or edible, oysters (genus Ostrea) lead sedentary lives attached to an underwater object. The shell is quite asymmetrical. The valve that is fastened to a submerged object is large and quite thick; the other one is smaller and thinner. The two parts of the shell are closed by a single muscle, popularly called the "heart," which extends from about the center of one valve through the animal's body to the other valve. True oysters occur in many parts of the world, but especially along the coasts of Europe, North America, and Japan.

When the first European settlers came to North America, they found that Indian tribes along the coast depended on oysters for a considerable part of their diet. Evidently they had been eating these mollusks for generations, because large piles of oyster shells had collected around Indian towns and encampments. The first settlers and those who followed picked and dredged oysters from the shallow bays. It was long thought that the supply was inexhaustible.

Increasing demand, however, led to overfishing in the late 19th century. It soon became necessary to supplement the natural supply by planting barren areas with young oysters, thus starting new beds. Today a considerable portion of the oyster supply in North America comes from privately owned beds. Oysters are also raised in Japan and in various European countries, particularly France and the Netherlands.

Successful oyster cultivation requires a familiarity with the life cycle of these shellfish. The female of a typical species, such as Ostrea virginica, an oyster found along the eastern coast of North America, produces millions of eggs each year. The eggs are discharged into the water, where many are fertilized by sperm cells ejected by the males. A fertilized egg develops into a tiny larva, which swims about freely for a few days and then begins to develop a shell. Within a week, the creature is entirely enclosed. It drops to the bottom, where it becomes attached to a rock or other solid object. The spat, as the young oyster is called, grows rapidly and in time becomes a mature oyster.

Despite the vast numbers of eggs produced by female oysters, the oyster population is not constantly on the increase. Many of the eggs are not fertilized; vast numbers of the little larvae are eaten by fish during their brief period of swimming. Even after they drop to the bottom and become securely attached, the oyster larvae are by no means safe. They may be suffocated by shifting sand and mud, or devoured by starfish, drumfish, or other natural enemies. And then, of course, once oysters reach the adult stage, they are sacrificed by the millions to meet the demands of the market.

During the breeding season, oyster cultivators locate places where the surface of the sea is covered with oyster larvae. The cultivators pave the seafloor of such places with various hard materials, such as old bricks, tile, empty bivalve shells, brush, and discarded metal parts. When the spat drops to the bottom, it becomes attached to the paving materials. These materials are then dredged up and planted in spots that have been selected as favorable for the development of oyster beds.

Oysters are often planted in moderately shallow water with hard-mud seafloors. In such places, there are likely to be marine plants, which will provide food for the microscopic organisms upon which oysters feed. Oyster cultivators avoid places where there is shifting mud and sand, or where starfish or other natural enemies of oysters abound, or where the waters may be subject to pollution.

Oysters that are ready for market are collected in shallow waters by means of oyster tongs  instruments akin to two long-tined rakes, hinged so as to open and close like shears. In deeper waters, the oysters are taken by means of a dredge.

In France, young oysters are removed to partially enclosed "growing" ponds where the tides are admitted through sluices and floodgates. When fully grown, the oysters are fattened in small enclosed ponds called claires.

Japanese oyster farms are generally placed in shallow, brackish water. Each farm is enclosed by a bamboo fence or hedge. The young oysters are collected and held on bamboo stakes thrust into the bed. When the oysters are fully grown, the stakes are pulled out and the oysters harvested.

Clams

Many of these bivalves are also eaten by people. One of the most sought after is the soft-shell clam, Mya arenaria, so named because of its rather thin and fragile shell. It is found in Europe and along the Atlantic and Pacific coasts of North America. The soft-shell clam is also called the long-necked clam because of the unusual length of its "neck"  actually, two tubular siphons joined together and covered with tough skin.

The soft-shell clam uses its tongue-shaped foot to burrow into mud or sand to a depth of 3 to 4 inches (7.6 to 10 centimeters). The "neck" extends just out of the sand at high tide, as the animal feeds. At low tide, small holes in the mud or sand betray where the clam is buried. It is then a simple matter for a clammer to walk along the beach and dig out the desired amount of the bivalves.

The hard-shell clam, Venus mercenaria, differs in various respects from the soft-shell variety. Its thick, solid shell is a rather dirty-white color marked with concentric rings. The inner part of the shell is whitish, turning to purple at the outer edges. In the Americas, this purple section was used by coastal Indians for the money known as wampum. The hard-shell clam is also known as the quahog and as the littleneck clam, since its siphons are much shorter than those of the soft-shell variety.

The hard-shell clam is found in great numbers along North America's Atlantic coast, where it dwells on sandy or muddy seafloors at depths ranging up to 50 feet (15 meters). The creature slowly moves through sand or mud using its large foot. Clammers usually go out in boats to fish for hard-shell clams, gathering them with a rake or dredge. The clams are served raw on the half shell or are used for clam fries and in chowders.

Perhaps the most remarkable member of the clam group is the giant clam, Tridacna gigas, found in the coral reefs of the Pacific. This is the largest known of the living bivalves. Its shell may be almost 3 feet (1 meter) long and weigh more than 440 pounds (200 kilograms), with the edible portion accounting for 20 pounds (9 kilograms) or more.

Scallops

The bivalves known as scallops are found in many parts of the world, with a range extending from shallow water to fairly deep water. The shell is fan-shaped, and the valves arched and rounded. Two winglike projections occur at either end of the hinge of the shell. About 20 ridges radiate from the hinge, increasing in width as they extend outward.

Scallops are good swimmers, especially when young. The jets of water they spout as they alternately open and close their shells propel them through the water in a series of jumps.

Several species are highly esteemed as food. One of the most common species is the Atlantic bay scallop, Aequipecten irradians, found along the eastern coast of the United States. The only portion of the body considered edible is the single large muscle that in life serves to hold the two valves of the shell together.

Mussels

The marine mussel has a wedge-shaped black or bluish shell. A tuft of threads, called a byssus, is secreted by a gland located immediately behind the foot. These threads harden when they come in contact with seawater, causing the animal to be firmly attached to solid objects such as rocks. The byssus can be discarded and a new one secreted. The animal can thus move to new surroundings if unfavorable conditions arise.

The blue or edible mussel known as Mytilus edulis is popular in various parts of Europe and the United States. It abounds in Atlantic coastal waters and in the Mediterranean Sea.

Recently the zebra mussel, Dreissena polymorpha, commonly a marine mussel, has reproduced at astonishing rates in the fresh waters of the Great Lakes. These prolific mussels have damaged water-supply systems and navigational buoys, and threaten native wildlife in the area.

Teredos

The teredo, or shipworm, excavates burrows in wood that is submerged in salt water. The two valves of the teredo have fine ridges, much like the teeth of a file. Soon after it hatches from the egg, the teredo begins rasping with its double file at the wood of a pile or ship bottom. As the burrow deepens, it is lined with a pearly coating. In time, the teredo becomes a long, wormlike creature, its tapering body dwarfing the tiny valves, which are at the innermost part of the burrow. Siphons protrude from the opening of the burrow to draw in water and food and force out wastes. When the siphons are drawn in, the hole is closed by means of two plates attached to the rear of the body.

Outwardly, a piece of timber attacked by teredos shows only a number of small holes. Inwardly, it may be honeycombed with teredo burrows, sometimes so close together that the wood between them is as thin as paper. In time, even the most-solid timbers become so burrow-riddled that they collapse. Metal or concrete sheathing protects timber from the attacks of teredos. Heavy impregnation with creosote has also proved effective.

Snails and Related Forms

Snails, slugs, limpets, abalones, and conches are included in the large class of mollusks known as gastropods. These animals have a foot and mantle cavity, like other mollusks. They also have a well-developed head region and generally sport a spirally coiled, one-piece shell.

Snails

Snails are ubiquitous. Some dwell in the ocean, others in the freshwater of rivers, ponds, and lakes. Land snails abound in tropical jungles and in damp temperate regions.

The snail's head bears the mouth opening and one or two pairs of tentacles. The eyes are set upon or at the base of the tentacles. The animal uses its flat foot to creep from place to place. Specialized gland cells of the foot secrete mucus, which lubricates the path over which the snail crawls. This accounts for the slick trail that the animal leaves as it passes over a more or less flat surface. Both the head and foot of a snail can be withdrawn into the shell.

Freshwater snails and land snails have been eaten by people since prehistoric times. Today they are still regarded as delicacies in many countries. The market supply comes largely from snails that are raised in captivity on special farms in southern France, Italy, and Spain. About 10,000 snails can be kept in a 100-square-foot (9-square-meter) pen, where they are fed meal, vegetables, and bran.

In many areas, snails are considered pests because they feed voraciously on garden crops. The giant African snail, Achatina fulica, has become a particularly serious menace. This creature, a native of East Africa but now found in many other lands, is sometimes more than 6 inches (15 centimeters) long and as big around as a tennis ball. Its diet is varied, including garden plants, flower petals, decaying tissues, and manure. It is long-lived and fertile and can thrive under the most unfavorable conditions.

Whelks and periwinkles are marine snails commonly used as food by Europeans. The whelk is widely distributed in the North Atlantic. Besides serving as food, it is used as bait in cod fishing. Periwinkles are found in temperate and cold seas in many areas. They abound on rocks and in seaweed, on which they feed. The long tongue, or radula, of the periwinkle is a remarkable structure equipped with many rows of sharp, curved teeth.

The rasping radula of the snail known as the oyster drill, Urosalpinx cinerea, is particularly well developed. This tiny creature, less than 1 inch (2.5 centimeters) long, drills a hole through the shell of an oyster near the hinge and then sucks out the soft body of the victim through the hole. The oyster drill is one of the chief foes of those who cultivate or fish for oysters.

Slugs

Among the snails' kin are the curious animals called slugs. These mollusks, which range in length from 1 to 4 inches (2.5 to 10 centimeters), have no external shell. Land slugs live in moist places, often under stones and in holes in the ground. At night, they emerge from their retreats to feed on plants, sometimes in a nearby vegetable garden. Sea slugs crawl on rocks or seaweed in shallow water along the coasts of North America, Europe, and Asia; they also feed on plant matter.

Limpets

The limpet has a rounded or oval shell that looks like a diminutive volcanic cone. Some limpets even have a small opening at the top of the shell, suggesting a crater! Thanks to their suckerlike foot, limpets adhere well enough to rocks near the low-water mark that they can withstand the beating of the surf. At high tide, they move about in search of the algae on which they feed. After their feeding forays, they again attach themselves to the rocks. Limpets are found in many parts of the world.

Abalones

The shell of the abalone has a rather startling resemblance to a human ear, leading to this gastropod's nickname of ear shell. The large shell is very ornamental, particularly after the rough outer surface has been polished. Abalones are found in the Far East and on the Atlantic and Pacific coasts of the Americas. They live on rocks near the shore, feeding on seaweed. When disturbed, they cling with surprising tenacity to rocky surfaces. The flesh is often used in stews and chowders, or is prepared in the form of a steak; in the Far East, it is generally dried or smoked.

Conches

The conch is a large gastropod especially common along the coasts of the southern United States and the West Indies. The shell—sometimes 10 inches (25 centimeters) long and weighing as much as 5.5 pounds (2.5 kilograms)  has a small spire with a large lower whorl. The foot of the conch is equipped with a clawlike appendage. The animal moves in a series of leaps, sometimes turning quickly to avoid capture. Conch shells are often made into horns, cameos, and buttons. The flesh is a popular food, particularly in chowders.

Tooth Shells and Chitons

The mollusk class of the scaphopods, or tooth shells, is a small one, numbering only about 200 species. In most species, the long, curved, tapering, ivory-colored shell looks something like a boar's tusk. In some varieties  those known as elephant-tusk shells  the shell is not curved. Tooth shells generally live in fairly deep coastal waters.

The chitons and their kin make up the class Polyplacophora. They are found everywhere except in the polar regions. Most chitons have a shell consisting of eight overlapping plates.

Neopilina is the sole surviving genus of the class Monoplacophora. Its shell is caplike, and its body shows segmentation, a primitive feature relating mollusks to the segmented worms.

Economic Importance

The mollusks are useful to humanity in a number of ways. Mollusk shells, for example, have many human applications. The beautiful shells of abalones, conches, and other varieties are commonly sold as souvenirs. The mother-of-pearl inner layer of various shells is used for pearl inlays and knife handles and in hundreds of other ways. Tons of bivalve and other mollusk shells are ground up every year and used as material for surfacing roads. Because of their lime content, the ground-up shells are used as fertilizer and are also fed to domesticated birds, such as chickens. Certain mollusks also produce pearls.